Shigellosis - Shigella

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Shigella, Gram-negative bacteria

Shigellosis, or bacillary dysentery, is an intestinal infection, especially of the colon, that is a major public health problem in many undeveloped and developing countries, and is the main cause of childhood diarrhea. The disease is characterized by the painful and frequent passage of stools that contain large amounts of blood, mucus and pus; followed by fever and stomach cramps.


Shigellosis is caused by a group of gram-negative, facultative intracellular pathogens, Shigella bacteria. The Shigella organisms were recognized in the 1890s as etiologic agents of bacillary dysentery. Shigella oraganims are of the Enterobacteriaceae family and tribe Escherichieae. There are a total of 4 species: Shigella dysenteriae type 1, Shigella flexneri, Shigella boydii, and Shigella sonnnei. They are also known as groups A, B, C, and D, respectively, and the groups relates to the severity and intensity of shigellosis caused. Shigella dysenteriae is also refered to as Shigella shigae.

Out of the four species of Shigella, S. dysenteriae and S. flexneri are the most common in developing countries and produce the most severe form of shigellosis. S. flexneri is refered to as endemic, meaning it is present at all times, inmost commununites, while S. dysenteriae occurs in an epidemic pattern. The S. dysenteriae bacteria, often is absent for a number of years, but then reappears and infects a large proportion of the population. The least virulent and most common Shigella bacterium is S. sonnei. S. boydii, on the other hand, is the least common form of Shigella, usually found in the Indian sub-continent, and causes disease of intermediate severity.

Disease Cycle

Life Cycle of Shigella.

Shigella infection is very common in areas where sanitation is poor. Humans are the natural reservoir of Shigella, although other primates may also be infected. A wide variety of foods may be contaminated with endogenous Shigella species, but no natural food products harbor the organisms.

The disease shigellosis is spread by fecal-oral transmission. Shigellosis is a communicable infections disease that indirectly infects using vehicles such as food, water, and other biological products. A quick guide to remembering the sources is feces transferred to food, fingers, feces, flies and sometimes fomites. Vectors like the housefly may spread the disease by physically transporting infected feces. Shigellosis can also be acquired through direct person-to-person contact, horizontal communication, by certain modes of sexual contact. Shigellosis is associated with lax sanitation, malnutrition, and crowding, and is spread epidemically in daycare centers, prisons, mental institutions, nursing homes, and military camps.

Not much of the Shigella bacteria are need for an infection to occur; the infectivity dose is extremely low. S. dysenteriae only needs about 10 bacilli to cause the clinical disease, whereas 100-200 bacilli are need for S. flexneri, or S. sonnei infection.

The incubation period of Shigella species is inversely proportional to the load of ingested bacteria and varies from 12 hours to 7 days but is typically 2-4 days.


Chromosomal-coded and plasmid-coded genes are the virulence factors in Shigella species. Virulent Shigella strains produce the disease right after invading the intestinal mucosa; it is rare for the organism to penetrate beyond the mucosa.

Molecular genetics have found that the characteristic virulence trait is encoded on a large (220 kb) plasmid. The plasmid is responsible for the synthesis of the polypeptides that cause cytotoxicity, the destruction and lysing of immune cells. Once Shigellae loses its virulence plasmid, the organism is no longer pathogenic.


Primary infection consists of the host inducing an acute inflammation which is accompanied by PMN, polymorphonuclear cell, infiltration. This results in massive destruction of colonic mucosa. In the Subepithelial tissues, apoptotic destruction of macrophages are occurring, allowing the survival of the invading Shigellae; inflammation also facilitates further bacterial entry.

Gross pathology consists of superficial ulceration, mucosal edema, erythema, friability, and focal mucosal hemorrhage involving the rectosigmoid junction.


Colitis due to Shigellosis

Symptoms usually develop about 1 to 7 days, with an average of 3 days, after a an individual comes in contact with the Shigella bacteria.

Minor symptoms include:

• Acute (sudden) abdominal pain or cramping

• Acute (sudden) fever

• Blood, mucus, or pus in stool

• Dehydration

• Crampy rectal pain (tenesmus)

• Nausea and vomiting

• Watery diarrhea

Shigella dysentery, the severe form may lead to a number of dangerous complications.

Severe symptoms include:

• severe anorexia (loss of appetite)

• hypoproteinaemia (a low concentration of blood protein)

• hyponatraemia (a low concentration of blood sodium)

• dilation of the large intestine

• seizures

• anaemia

• kidney damage

• persistent diarrhoea

• weight loss and malnutrition


The global alert and reponse (GAR) by the World Health Organization, WHO, shigellosis has been recorded on:

1. 14 July 2004 - Shigellosis in Sudan

2. 5 November 2003 - Shigellosis in Central African Republic

3. 5 November 2003 - Bloody diarrhoea in Liberia

4. 4 February 2000 - Dysentery in Sierra Leone

5. 28 January 2000 - 2000 - Dysentery in Lesotho

6. 24 January 2000 - 2000 - Dysentery in Sierra Leone


Exams and Tests that are done to diagnose shigellosis are dehydration with fast heart rate and low blood pressure, abdominal tenderness, elevated white blood cell count, stool culture, and white blood cells in stool.

Diarhea symptoms usually last 2 to 7 days; during that time electrolytes, salt and minerals, and fluids are lost. The goal of treatment is to replace most of the fluids that has been lost. Some medication that prevent diarrhea are generally not given because they may prolong the course of the disease. Antibiotics are given to help shorten the length of the infection and help prevent the bacterium from spreading to others. Patients with severe shigellosis may be prescribed antibiotics such as trimethoprim, ampicillin, ciprofloxacin, sulfamethoxazole, and azithromycin.

Related Diseases/Sequelae

Shigella can establish a chronic carrier condition in some people that lasts several months.

People, who have had shigellosis from Shigella flexneri, later develop post-infectious arthritis. This is characterized by pains in the joints, irritation of the eyes, and painful urination. It can last for months to years, and can lead to chronic arthritis. Only people who are genetically predisposed to post-infectious arthritis can acquire the reaction to the Shigella infection and the later symptoms.


Kathleen, Talaro. Foundation in Microbiology. 6th. New York, NY: McGraw-Hill, 2008. 615-616. Print.

Centers for Disease Control and Prevention." Shigellosis. N.p., n.d. Web. <>.

"Medline Plus." Shigellosis. N.p., 5/25/2010. Web. <>.

"Rehydration Projection." Shigellosis. N.p., n.d. Web. <>.

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